1. Field of the Invention
This invention relates to class-D amplifiers of Bridged Tied Load (BTL) output types, which use filter coils and low-pass filters.
2. Description of the Related Art
FIG. 1 shows a typical example of a class-D amplifier of the BTL output type, which acts as an amplifier for driving speakers of two channels (i.e., right and left channels). That is, a class-D amplifier 1 of the BTL output type comprises pulse-width modulators 10 and 11 that produce pulse-width modulated (PWM) signals whose pulse widths match input signal levels, as well as drivers 12 to 15 for driving switching elements (not shown) such as metal-oxide semiconductor (MOS) transistors connected between the positive and negative voltage supplies (not shown).
There are provided two-channel speakers 20 and 21, which act as loads for the class-D amplifier 1. In addition, there are provided four low-pass filters (i.e., LC filters), each consisting of a pair of a capacitor (or capacitance) and an inductor (or inductance), respectively connected with output terminals of the class-D amplifier 1. That is, the speaker 20 is connected with the class-D amplifier 1 via first and second low-pass filters, wherein the first low-pass filter consisting of a capacitor C1 and an inductor L1 is connected with the output of the driver 12, and the second low-pass filter consisting of a capacitor C2 and an inductor L2 is connected with the output of the driver 13. In addition, the speaker 21 is connected with the class-D amplifier via third and fourth low-pass filters, wherein the third low-pass filter consisting of a capacitor C3 and an inductor L3 is connected with the output of the driver 14, and the fourth low-pass filter consisting of a capacitor C4 and an inductor L4 is connected with the output of the driver 15. All of these low-pass filters are provided to eliminate carrier frequency components included in the outputs of the class-D amplifier 1.
The drivers 12 and 14 receive pulse-width modulated (PWM) signals that are produced by the pulse-width modulators 10 and 11 respectively. In addition, the drivers 13 and 15 receive ‘inverted’ PWM signals output from the pulse-width modulators 10 and 11 respectively.
Therefore, each of the drivers 12 and 14 provides a positive or negative voltage in response to the polarity of the PWM signal, while each of the drivers 13 and 15 provides a positive or negative voltage in response to the polarity of the inverted PWM signal. Herein, the supply voltage (not shown) is inverted in polarity between a pair of the drivers 12 and 14, and a pair of the drivers 13 and 15. Thus, voltage pulses whose amplitude is double of the amplitude of the supply voltage are applied to the loads, namely, the speakers 20 and 21 respectively. In FIG. 1, arrow symbols represent flow directions of currents with respect to the loads 20 and 21 in response to the positive and negative polarities of the BTL outputs of the class-D amplifier 1.
The class-D amplifier 1 of the BTL output type uses two low-pass filters for eliminating carrier frequency components in the positive and negative outputs with respect to a single channel. Therefore, four low-pass filters are used in total with respect to two channels, that is, right and left channels.
In the aforementioned configuration, the class-D amplifier requires four filter coils independently for use in four low-pass filters for eliminating carrier frequency components when driving two-channel speakers.
It may be possible to use a common-mode coil, an example of which is shown in FIG. 8, wherein plural coils having the same winding direction are incorporated and arranged in such a way that external lines thereof are partially crossed with each other. Specifically, FIG. 8 shows two coils 80 and 81 having the same winding direction, which are incorporated and arranged adjacent to each other along axial directions thereof within a package 90.
In FIG. 8, ends of the coil 80 are connected with terminals 91 and 92, and ends of the coil 81 are connected with terminal 93 and 94. Lines 95 and 97 are wired in connection with the terminals 91 and 92 for the coil 80, and lines 96 and 98 are wired in connection with the terminals 93 and 94 for the coil 81. Herein, the lines 96 and 98 for the coil 81 are crossed with each other.
Since the aforementioned class-D amplifier requires four coils independently for use in the four low-pass filters for eliminating carrier frequency components of PWM signals corresponding to BTL outputs thereof, there are problems in that a considerable amount of the cost should be incurred for the provision of the four independent coils, which inevitably increase installed areas therefor.
In addition, flow directions of currents are reverse to each other between the positive polarity and negative polarity of the BTL outputs of the class-D amplifier. Therefore, in the package incorporating plural coils each having the same winding direction, it is necessary to cross external lines in the substrate wiring with respect to at least one coil in order to avoid occurrence of inductive coupling between the coils. This makes the substrate wiring to be complicated. In addition, it is difficult to provide countermeasures against unwanted radiation, which is troublesome.